1. Technical Field
This invention relates generally to protocols and, in particular, to acquisition of global positioning signal.
2. Related Art
The global positioning satellite (GPS) system is a satellite based navigation system having a network of 24 satellites, plus on orbit spares, orbiting 11,000 nautical miles above the Earth. Each satellite in the system orbits the Earth every twelve hours.
A prime function of GPS satellites is to serve as a clock that keeps GPS time. Each satellite derives a signal from an on board 10.23 MHz Cesium atomic clock. GPS time is kept in terms of seconds and weeks since Jan. 6, 1980. There are 604,800 seconds per week. Consequently, GPS time is stated in terms of a time of week (TOW) and a week number. TOW ranges from 0 to 604800. The week number started with week zero and is currently in excess of 1000 weeks. The TOW can have a fractional part, such as in the real time clock, where the resolution is 1/32,768th of a second. GPS time is fundamental to the GPS system. At each GPS satellite, the time of transmission of each chip is controlled down to a few nanoseconds. Consequently, knowledge of precise GPS time allows one to know exactly what chip of a satellite's waveform is being transmitted at any given time.
Each satellite transmits a GPS spread spectrum signal having an individual pseudo noise (PN) code. By transmitting several GPS signals over the same spectrum with each GPS signal having distinctly different PN coding sequences, the satellites may share the same bandwidth without interfering with each other. The PN codes used in the GPS system are 1023 bits long and are sent at a rate of 1.023 megabits per second, yielding a time mark, called a “chip” approximately once every micro-second. The sequence repeats once every millisecond and is called the course acquisition code (C/A code). Every 20th cycle the C/A code can change phase and is used to encode a 1500 bit long frame of data that contains a precise orbital description for the transmitting satellite, called ephemeris data, and approximate orbital descriptions for all satellites in orbit, called almanac data. The ephemeris data repeat each frame, while the almanac data are distributed over 25 frames before repeating. Various other data are also included in the overall frame structure.
There are 32 PN codes designated by the GPS authority for use in orbiting satellites. Additional codes are designated for other purposes. Twenty-four PN codes are assigned to current satellites in orbit. The remaining PN codes are spare codes that may be used in new satellites to replace old or failing satellites. A GPS receiver may, using the different PN code sequences, search the signal spectrum looking for a match. If the GPS receiver finds a match, then it is able to identify the satellite that generated the GPS signal.
GPS receivers may use a variant of radio direction finding (RDF) methodology, called triangulation, in order to determine the position on the Earth of the GPS receiver. The position determination is different from the RDF technology in that the radio beacons are no longer stationary; they are satellites moving through space at a speed of about 1.8 miles per second as they orbit the Earth. By being spaced based, the GPS system can be used to establish the position of virtually any point on Earth using a triangulation method.
The triangulation method depends on the GPS receiver units obtaining a time signal from multiple GPS satellites enabling the distance to each satellite to be calculated. If, for example, the GPS satellite is 11,000 nautical miles from the GPS receiver, then the GPS receiver must be somewhere on a location sphere having a radius of 11,000 nautical miles from the GPS satellite. When the GPS receiver ascertains the position of a second GPS satellite, then the GSP receiver calculates its location based on a location sphere around the second GPS satellite. The possible location of the GPS receiver on the two spheres lies at there intersects and forms a circle. To further resolve the location of the GPS receiver, the distance from a third GPS satellite to the GPS receiver is determined to be a location sphere around the third GPS satellite. The location sphere of the third satellite intersects the location circle produced by the intersection of the location sphere of the first two GPS satellites at just two points. By determining the location sphere of one more GPS satellite, whose location sphere will intersect one of the two possible location points, the precise position of the GPS receiver is determined. As a consequence of the GPS system, the exact time may also be determined, because there is only one time offset that can account for the positions of all the satellites. The triangulation method may yield positional accuracy on the order of 30 meters, however the accuracy of GPS position determination may be degraded due to signal strength and multipath reflections of the satellite signals.
GPS receivers may have visibility of as many as 12 GPS satellite signals at one time at the surface of the earth. The number of orbiting satellites that are visible depends on the location of the receiver and the locations of the satellites at a given point in time. The number visible in an unobstructed location may vary from approximately 5 to 12 satellites. In certain environments such as a canyon, some GPS satellites may be blocked out, and the GPS position determining system may depend for position information on satellites that have weaker signal strengths, such as GPS satellites near the horizon. In other cases overhead foliage may reduce the signal strength of the GPS satellites that are received by the GPS receiver unit. In either case the signal strength is reduced.
There are multiple ways of using radio spectrum to communicate. For example in frequency division multiple access (FDMA) systems, the frequency band is divided into a series of frequency slots and different transmitters are allotted different frequency slots. In time division multiple access (TDMA) systems, the time that each transmitter may broadcast is limited to a time slot, such that transmitters transmit their messages one after another during an allotted period. Furthermore, the frequency upon which each transmitter transmits in a TDMA system may be a constant frequency or may be continuously changing (commonly referred to as frequency hopping).
A third way of allotting the radio spectrum to multiple users is through the use of code division multiple access (CDMA) also known as spread spectrum communication. In a CDMA system, all users transmit on the same frequency band all of the time. Each user has a dedicated code that is used to separate his transmission from transmissions from other users. This dedicated code is commonly referred to as a spreading code, because it spreads the information across the band. The code is also commonly referred to as a Pseudo Noise or PN code. In a CDMA transmission, each bit of transmitted data is replaced by a particular spreading code associated with a user. If the data to be transmitted is a binary “1”, then the particular spreading code is transmitted. If the data to be transmitted is a binary “0”, then the spreading code is replaced by the inverse of the spreading code.
To decode the transmission at the receiver it is necessary to “despread” the code. The despreading process takes the incoming signal and multiplies it by the spreading code and sums the results. This process is commonly known as correlation, and it is commonly said that the signal is correlated with the PN code. The result of the despreading process is that the original data may be separated from all other transmissions, and the original signal is recovered. A property of PN codes used in CDMA systems is that the presence of one spread spectrum code does not change the result of the decoding of another code. The property that one code does not interfere with the presence of another code is often referred to as orthogonality, and codes that possess this property are said to be orthogonal codes.
The process of extracting data from a spread spectrum signal is commonly known by many terms, including correlation, decoding, and despreading. The codes used by a spread spectrum system are commonly referred to by a variety of terms including, but not limited to, PN (Pseudo Noise) codes, PRC (Pseudo Random Codes), spreading codes, despreading codes, and orthogonal codes.
It is because CDMA spreads the data across the broadcast spectrum that CDMA is often referred to as spread spectrum. The global positioning system (GPS) uses spread spectrum technology to convey data to ground units. The use of spread spectrum is especially advantageous in the GPS systems. Spread spectrum technology enables GPS receivers to operate on a single frequency, thus saving on additional electronics that would be needed to switch and tune to other bands if multiple frequencies were used. A spread spectrum system also minimizes power consumption requirements compared to other types of radio systems. GPS transmitters for example require 50 watts or less and tolerate substantial interference.
A GPS receiver determines its position by measuring the range to four or more satellites. Since the positions of the satellites are known by evaluating their orbital equations, the location of the receiver can then be computed. The orbital equations are based on parameters broadcast by the satellites. For precise position, the more precise ephemeris parameters are required. In order to determine the range to a satellite, knowledge of GPS system time must be known, because the orbital equations are a function of time. The satellites orbit the earth about twice per day. The normal procedure is to use synchronization and time data provided by the GPS signal from each satellite. Using this procedure requires at least 6 seconds because the synchronization is repeated one time in each 6-second sub-frame of the GPS signal structure.
The use of data collection from the satellite to resolve GPS time requires at least 6 seconds to decode the required data to provide a common time error for all of the satellite signals being received. When the initial time and position data are accurate to better than ±0.5 PN code period (±0.5 ms), the common value of time error for all satellites can be computed without the need to obtain this data from the satellites' signal structure. Advances in digital integrated circuit technology allows acquisition of the PN codes of several satellites in less than a second rather than the tens of seconds requires a few years ago. Hence the minimum six second penalty to establish a common time error by the conventional approach is now the dominant time delay in realizing the initial position solution. In many applications, it is critical to provide position updates with minimized expenditure of energy from a battery. This can be done most efficiently if most circuitry of the receiver can be turned off between position updates. With the circuitry off, synchronization with the received waveforms of the various satellites is lost and has to be reestablished for the next position update. What is needed in the art is an approach to maintaining time between updates with low power consumption penalties and an approach to resolving code-period range ambiguity to a GPS satellite system in tens of milliseconds, rather than several seconds.